Brass is a material involving many opportunities and fields of application. The basic constituents are copper (Cu) and zinc (Zn). By additives of different alloying material such as i. a. lead (Pb), tin (Sn), iron (Fe), aluminum (Al), nickel (Ni), manganese (Mn), silicon (Si) and/or arsenic (As) the brass can be given unique properties and there are many different brass qualities for different types of machining and end products. Brass may as well involve antimony (Sb), phosphorous (P), boron (B) and/or sulfur (S).
Brass can be made in the form of bars, profiles and blooms being semi-finished products to be further refined. Samples of such end products are screws, nuts, water and sanitary armatures, lock details, electric components, ornamental objects etc. Above all brass is a closed cycle material having its given place in an environmental promoting workshop production. Brass is profitable to be recovered and therefore almost 80 percent of the raw material is in the form of brass scrap, partly as waste material from the workshop industry and partly from recovery enterprises.
The percentage 0.2 of Pb is obtained from the definition of the so called Hygienic Copper Alloy Composition List, of lead free brass. Alloys of brass and other metals and materials being in contact with drinking water are controlled by this list and will be valid from Dec. 1, 2013 in those countries which have signed the 4MS, (Four Member State), declaration, a work being an extension of the previous EAS (European Acceptance Scheme), work started in 1997 and being sanctioned by the EU-commission. The target with the 4MS declaration is to create a common directive for all the 27 EU countries. Moreover there are similar regulations of the Pb percentage in brass alloys in other countries like the USA. The main difference between USA and Europe is that in the USA one is focused on restrictions of lead in separate articles (the average value being max 0.25% by weight Pb) while in Europe it is focused on the restriction of lead in the drinking water as such. The value allowed in the drinking water as such is higher in the USA than in Europe, 15 and 10 μg/l respectively [1]. Samples of brass alloys meeting the requirements of being defined as lead free brass are CW511L and EcoBrass® [1, 2].
In connection with these environmental demands on the precipitation of Pb in drinking water there is also the demand for eliminating Pb in the material as such. These work is in progress through different governmental stipulations but also on voluntary basis by so called environmentally classified systems. As an example in Sweden one can mention the Building Material Assessment (Byggvarubedömningen) and Basta, where lead free alloys are a demand.
The brass alloys with the EN-number CW614N and CW617N are two of the most common brass alloys for cut machining and forging [3]. For instance these alloys are used for water and sanitary armatures, oil and gas armatures as well as for many different details at the electric, engineering and car industry. The alloys are easy to polish and to surface for having a very high surface finish. The CW614N comprises 39% by weight Zn, 3% by weight Pb and the rest is Cu and thus has the composition designing CuZn39Pb3. The CW614N is also referred to as a free-cutting brass as it is used for automatic machining, and CW617N is used for hot forged details.
By adding lead to brass alloys such as the CW614N the machinability is enhanced. A small part of 0.2% by weight is dissolved, the lead atoms are much larger than the copper and zinc atoms and due to their size they lock the dislocation movements. This enhances among others the chip breaking being of great importance. The rest forms a lead-copper phase being precipitated at the grain boundaries. This phase melts at the temperatures prevailing in the cut zone and the molten metal acts as a lubricant during the cut progress. By lowering the Pb below 0.2% by weight one obtains a very deteriorated machinability generally seen.
The part of the lead-copper phase being precipitated at the grain boundaries will be a part of the surfaces of the work piece by the cutting machining. The phase is more and easier stretched out than the remaining parts due to the low strength and high ductility, it may also be liquid. These surfaces will be found in products/components, water taps, being in contact with drinking water. In this way lead may be leached to the water and have an injurious effect on our health.
Another aspect is that the brass may be dezincificated by intergranulated corrosion (4) and thereby expose the remaining grain structure. A minimal addition of Pb is favorable since also these grains can be in contact with water.
However, the absence of a lead-copper phase at the grain boundaries impairs the machinability of a copper alloy. The main difficulties with machining include:
1. Deteriorated chip breaking and chip control
2. Chip widening, the chip expands sideways, see FIG. 1
3. Burr formation
4. Build up edge, “BUE”, on the cutting tool rake face, which subsequently ends up on the workpiece surfaces
5. Significantly higher cutting forces
6. Vibration tendency is significantly higher due to higher cutting forces in the chip thickness direction, see FIG. 2.
Thus there is a great need for an improved brass alloy with significantly less addition of lead Pb without impairing machinability.